[FLOC-1013] Add an Elasticsearch connection test to the Logstash startup script

Here's a version of the logstash image which checks that a connection can be established to elasticsearch before starting logstash.

I switched the entrypoint script to python to make things clearer.

I also now exec the logstash binary rather than forking so that logstash can receive signals from docker ...which allows it to be more easily killed when launched as a foreground docker process.

I tested this by 
 * building the image `docker build --tag clusterhq/logstash `
 * running the image in the absence of elastic search `time docker run --interactive --tty --env=ES_PORT_9200_TCP_ADDR=10.0.126.72 --env=ES_PORT_9200_TCP_PORT=9200 clusterhq/logstash`
 * running it after starting elastic search `docker run -p 9200:9200 clusterhq/elasticsearch`
 * running it after dropping port 9200 packets `sudo iptables -I INPUT -p tcp --dport 9200 -j DROP`
 * running it and then starting ES 10s later.

It seems to work.
It probably doesn't need an extension.